While the out-of-universe reason for so many of the main characters' Earth-X versions to be evil is obvious, it also makes a lot of sense if you think about it.
Oliver and Tommy: Assuming their pasts are otherwise similar to on Earth-1, both are the sons of wealthy oligarchs, meaning that their fathers likely hold or held a lot of power, and would be able to use that to ensure the best training and positions for their sons. Alternatively, even if that's not true, Overgirl does say that Earth-X is a meritocracy. And we already know that the Queens and Merlyns are very good at fighting.
Kara being Overgirl may seem odd at first, considering how she is usually so driven to help others. But, as Season 3 has repeatedly shown, Kara also has a very powerful sense of pride and of being above everyone and everything else, even with how she was raised. Earth-X would do nothing but encourage that view, meaning that instead of merely suppressing her emotions it's very easy to see how a Nazi-raised Kara could begin to see herself as an Übermensch. In addition, Kara is basically the exact vision of the Nazi ideology's perfect being: tall, powerful, and blonde (although she does have grey-green rather than blue eyes). And while it might seem counter-intuitive that an ideology based on racial purity would accept a literal alien, historically the Nazis actually did make exceptions for non-Aryans they deemed useful or who could further their goals. Eventually stated outright by Overgirl.
Overgirl:to Supergirl Oh please, we're everything they want to be. Blonde, white. Aryan perfection...you're a god to them.
It's pretty obvious that there's no Kal-El on Earth-X (or at least none that anyone knows about). Without Clark being a role model regarding how to treat people without power, Kara could quite easily have taken to the mindset of them being lesser.
Keep in mind Clark was such a role model to Kara because of how he was raised on their Earth. Earth-X's Kal-El could have still come to Earth, and it would have been as a baby, and therefore he would have been raised in the Nazi ideology from the beginning. It's entirely possible that he was the one who managed to convert the teenage Kara to an ideology antithetical to how she was raised on Krypton.
Could have even been a Red Son situation, only in Nazi Germany instead of the Soviet Union.
There's also the idea that Superman, and by the metrics of this series Supergirl, represent the very best of the ideals they were raised with. In Prime universes, that's good old-fashioned middle American values. In a world ruled by Nazis, that's. . . well. . .
The heroic characters who stay heroic also make sense. James Olsen, being a black man, would never have a chance in the Nazi regime so of course he would still be raised to believe in the old American ideals. In the case of Leonard Snart, the man was a rebel by nature so when the natural order is evil, damn right he'd be a goody-two shoes. This may also account for Leo exhibiting more Camp Gay behavior than his Earth-1 version. He is rebelling against one of the most homophobic regimes imaginable and likely defines himself by being exactly what the Nazis condemn.
The diverse characters of the Arrowverse gives the ensemble of Heroes and Legends one of the strongest reasons to utterly despise Nazis any superhero team has ever had, and provide a convenient explanation as to why there was no attempt at convincing any of the Earth-1 people to change sides: of the total of 21 heroes, Mission Control team members, and Mick from Earth-1, Earth-38, and Earth-X who appear in the crossover, only Mick, Ray, and Harry don't have an automatic personal reason to have complete contempt for Nazism. The rest are Jewish, non-"Aryan", gay/bisexual, or currently in a relationship with or have a family member who is.
Continuing a joke from last year's crossover, "Invasion!", in Part 3 of Crisis, Kara tells Thawne, who is about to cut Kara up, that her cousin will find him. In Part 4, Ray (played by Brandon Routh) comes to save her.
Professor Stein all but begging Jax to sever their connection in order to save the latter has a double meaning. Stein could do nothing to save his first partner and Jax's predecessor, Ronnie Raymond. This time, he has the chance to save Jax, and he will do anything to make Jax live, even at the cost of his own life.
Susanna Thompson is the voice of Gideon-X. Only right that Oliver-X would have his mother's voice as the computer on the Waverider-X.
About half of the Legends and most of Team Arrow spend the bulk of the crossover elsewhere or imprisoned. The only heroes who play a major role throughout are the ones who have a personal connection to Barry and/or Iris, i.e, the ones who'd logically be invited to their wedding. Aside from Supergirl's sister Alex, at least.
Not really, Sara does not know Barry on a personal level while Ray (the Atom) does.
So do Diggle and Lyla.
When Nora West Allen, Barry's and Iris' daughter from the future talks to Barry at the wedding, the Flash Leitmotif plays, hinting at their connection.
Martin being on the receiving end of Retirony instead of peaceful retirement. If Stein simply retired he would still probably be required to make sporadic appearances on The Flash, since he and his family live in Central City. On a meta level, this wouldn't work because of Victor Garber's decision to leave the franchise.
Earth-X has a noticeable dearth of metahumans; the only ones we see (the Ray and the Reverse-Flash) are both from Earth-1. Considering the strict control a fascist regime would have over the sciences, it's quite likely that the particle accelerator never exploded on their world, limiting the metahumans to individual accidents like Livewire. This also explains why they needed a dimensional gateway; they didn't have anyone with breacher powers like Vibe or Gypsy.
While Kara is somewhat squicked about the idea that her doppleganger is married to Oliver-X, it makes perfect sense after the denouement of last year's crossover. Felicity mentions how much she and Kara look alike, "almost like looking in a mirror." Since Felicity is Jewish, Oliver-X would never consider her counterpart worthy, but since Kara-X is there, he has she would be his type.
Winn-X seems on a very short fuse during his entire encounter with the heroes, adamant about destroying the gateway between earths and nearly killing them all to keep his enemies away from his home. At first it seems puzzlesome even given its his Earth-X doppleganger, but then I realized: on Earth-38, Winn and James are best friends. If there's anything of that relationship shared by their counterparts fighting the oppressive Nazi regime, then not only did the resistance lose one of its best champions, but Winn lost a friend. No wonder he's so pissed.
Mick responding to Snart's comment on due process with "Never heard of it" may seem like a throwaway joke or a reference to his being a convicted felon, but...in this time, in this situation, Mick was right. They were facing an invading army bent on conquest and slaughter. Due process has no place in a literal war zone. Lethal force is entirely justified and, dare say, actively encouraged, especially for those, like Mick, who don't have any way to reliably incapacitate-it's kinda hard to shoot to wound using a flamethrower. In such a situation, the only ones who can really afford to uphold the "no killing" rule are those like Barry, Kara, Frost...
It is very heavily implied, given Stein correctly guessing that Earth-Xs Point of Divergence is the Nazis getting the Bomb, and the later appearance of the Nazi-fied Waverider, that the Legends only barely prevented the Earth-X timeline.
Not really. That same episode also points out that the Allies still would've won the war even if New York was destroyed, it just took two years longer.
That's because they only had the one bomb, supplied to them by Thawne and Darkh, but didn't actually crack the formula on how to make more. Once they used their one shot on New York, the war continued conventionally. Here they invented it on their own and could use them wantonly.
Thawne's last taunt to Barry before he leaves is to wonder what face he will be wearing the next time they meet. Thawne is ever growing closer to his utter bastard of a mainstream counterpart, if he hadn't crossed it already. And even worse, if you remember what happened to Earth-1 Wells just because Thawne wanted to impersonate him, then Thawne simply said he's willing to do such a horrific operation again, and again, and again.
While it was a funny juxtaposition that the Earth-X version of Mick Rory died saving cops from a burning building, wouldn't those cops have been part of the Nazi Regime?
Somewhat unlikely. You'd think that Citizen Cold (a gay man) would lament that his best friend sacrificed his life for people that didn't deserve it and note this to Earth-1 Mick.
With the exception of James Olsen, there are no confirmed doppelgängers of any of the shows' non-white heroes/characters. Given that their world is ruled by nazis, it isn't much of a stretch to think they were executed in the name of "genetic purity".
Its great possibility since White Canary was executed for being bisexual, but given Earth-X Felicity still lives despite the Holocaust never stopping in this world, the others may have survive unless until said/shown otherwise.
J'onn J'onzz came to earth before World War II, so in Earth X, he either escaped the white martians who killed most of his family only for his new home to be taken over by the white martians' earth equivalents, or he escaped the white martians only to be killed by Nazis. Or perhaps he fled the planet as soon as the Nazis took over.
Considering the only experience Oliver had with Kryptonians was Kara herself, and he was never introduced to Kryptonite (in fact, Barry never was either), why does he even have any? Ostensibly it could have been synthesized, but why does he even know about it barring that oneexplanation?
The last two times Kara has come to Oliver's universe Kara was 1) brainwashed by an alien and almost killed Oliver and 2) was trapped in her own mind by villain along with Barry it makes sense Oliver would want a plan to deal with her. As for where he got it, while J'onn got rid of the Kryptonite from the DEO, it is entirely possible Kara knew where to get some and convinced Clark to let her give a piece to Oliver just in case she is ever on another world where he or J'onn aren't there to save her. The Legends are another possibility as well.
Except that Kara was shocked to see Oliver use a Kryptonite arrow, so no one on her Earth could've helped him. As for the Legends, it's the same issue in that they were never even told about Kryptonite, and before the crossover, everyone was under the assumption that Kara, and by extension, Kryptonians, didn't exist on Earth-1.
Remember, Cisco can breach. It's entirely possible Oliver just asked Cisco if he knew of or could reach any place with Kryptonite.
Oliver probably realized how utterly, hilariously outgunned he was faced with a Kryptonian. With Barry and other metahumans he's dealt with, he and the team could come up with some method of dealing with them, but Kara was completely beyond anything Earth-1 had to offer, with no known weaknesses except alien mind-control. If he figured out there was a possibility there was a Krypton in his universe and there was no way anyone would be able to deal with them, he could very well have asked had Cisco to do some research, and Cisco might have asked Gypsy and Harry if they knew anything about Kryptonians anywhere in the multiverse, which would have led to Kryptonite.
Yeah, but wouldn't it be Out of Character for Cisco to try and figure out a way to kill one of their friends just in case they turn bad? The exact line of thought that let Snart become Captain Cold?
Really? Who came up with Babel Protocol for the Flash suit?
That's exactly why Cisco would have done something like that. If not Kara, Evil Dopplegangers aren't something he's naive about considering he's fought one himself, and they'd dealt with Kara being brainwashed herself. Also, who said anything about kill? Oliver's first shot was to her shoulder, not her heart.
Fair enough, but then we still run into the issue of Kara never mentioning Kryptonite the last two times she was on Earth-1. Even if it happened offscreen (for whatever reason), everyone was under the impression that nothing on Earth-38 existed on other Earths, hence their surprised reactions to Overgirl being an evil Kara. Oliver wouldn't have thought about Kryptonite being on other Earths except Kara's, and he couldn't have gotten it from there because J'onn and Superman got rid of all the Kryptonite in the beginning of season two.
He wouldn't necessarily been thinking of kryptonite specifically, but looking for a weakness, any weakness, and through that eventually learned about kryptonite.
Oliver had no reason to even believe that Kara was weak to Kryptonite.
Here's a plausible course of events. Oliver decides they need a safeguard against Kara. He asks Barry what Kara is. He then asks someone with knowledge of the multiverse, likely Cisco or Wells, if they can track down anything that can fight Kryptonians. Cisco or Wells does some trans-universal research and comes back with Kryptonite.
Everyone in the group seems to be both familiar with Krypton the planet and Kryptonite itself. Harry mentions there's 53 Kryptons (or was since many of them likely blew up like usual) like there's 53 Earth's, and Felicity's reaction to seeing the kryptonite arrow is to ask if Oliver shot Kara suggesting she knows exactly what it is and what it does. They also seem to be aware of Superman, so clearly at some point they were filled in in the basics of how her universe operates. How exactly Oliver got it is a good question since he's never been to her earth yet, but he clearly knows the gist of how Kryptonian's work.
Why is Eobard Thawne alive again? Because Caitlin killed the Black Flash! Might be reaching a bit, but better than 'cause Time Travel, dude!. Maybe Caitlin really did kill him off for real and thus all of his actions, like erasing Thawne from existence, were undone.
Or maybe the Speed Force brought him back as a sign of its disapproval of Barry escaping it.
The Thawne the Black Flash killed was from Doomworld it's possible when he went back in time he told the him that was already there to run away in case he fails.
But when the Black Flash erased Thawne, all the other time remnants he created disappeared, too. So wouldn't that Thawne be erased as well?
The Thawne Black Flash killed was the Thawne from the pre-Flashpoint timeline. This Thawne is the one from the current one.